The old task of distinguishing diamonds look-alikes from natural diamonds continues to be important. However this task becomes ever more difficult as quality of synthetic gems, which are partially indistinguishable from natural diamonds with the naked eye, improves and their characteristics, which could be used for the purpose of distinguishing them from natural diamonds, approach those of natural diamonds.
Two techniques, which are usually used in portable diamond testers, are based on diamonds' exceptionally high thermal conductivity and their high electrical insulation ability. Those diamond look-alikes that have similar electrical insulation ability, such as cubic zirconium, have low thermal conductivity, while those, which have thermal conductivity close to that of diamond, such as silicon carbide popularly known as moissanite, are not such good insulators. Therefore, successful diamond testers combine these two techniques for reliable detection of a wider spectrum of look-alike gemstones.
As technology progresses, moissanites with even smaller electrical conductivity are becoming available. There are gems on the market with electrical resistance in excess of 100,000 Megohms. Discriminating such weak conductors from a good insulator, like diamond, using a portable battery operated device is difficult, and even the best available testers cannot do it reliably.
It has been noticed that electrical conductivity of a moissanite gem can vary widely and even may be affected by the electrical current that passes through the gem during previous tests. Consequently, for a reliable test result it is desirable to subject the gem under test to as little and as short electrical disturbance as possible.
In order to be able to do measurements of the objects with so high resistance it is necessary to increase sensitivity of the measuring instrument. This aggravates the problem of picked AC interference, which depends on the environment and is practically unavoidable when such instrument contacts electrically with body of the user. So, while useful measurement signal for highly resistive gems goes down, interfering AC signal may become a dominating part of the signal if the measuring circuit is not designed carefully and/or adequate signal processing is not implemented.
Applying up to a 1000 V indiscriminately to a gem under test, as it is done in existing diamond testers, is not always necessary and can be counterproductive. There is also an issue of operator's safety and comfort if the operator's body becomes a part of the high voltage circuit, whether accidentally or by design. A possibility of unpleasant, let alone harmful, electrical shock under any scenario must be eliminated.